1. Field of the Invention
The present invention relates to an AF evaluation value calculating device for calculating an AF evaluation value used for AF (auto-focus) control of a digital camera.
2. Description of the Background Art
In the AF control of a digital camera, so-called “hill-climbing method” is generally employed. The hill-climbing method is a method of using, as a focus position, a lens position at which the maximum AF evaluation value among AF evaluation values in captured images at different lens positions along the optical axis direction is obtained. A digital camera of this kind therefore has an AF evaluation value calculating device for calculating an AF evaluation value.
Usually, the AF evaluation value is calculated by integrating contrast values of pixels in an AF area which is set in an image capturing area, so that the AF evaluation value calculating device has an accumulator for integrating contrast values. A conventional AF evaluation value calculating device has accumulators for integrating contrast values of the number equal to the number of AF areas which are set in the image capturing area.
In the AF control, the maximum AF evaluation value among AF evaluation values in lens positions is calculated, that is, the AF evaluation values are compared with each other, so that the AF evaluation values in the lens positions have to be transmitted and stored to a memory. In the conventional AF evaluation value calculating device, transmission of the AF evaluation values to the memory is controlled by a CPU on a digital camera side which performs AF control.
Conventionally, the number of AF areas which are set in the image capturing area ranges from one to about five at the maximum. FIGS. 8 and 9 show examples in each of which four AF areas A00, A01, A10 and A11 are set in the image capturing area. Since the number of AF areas is conventionally small, in the conventional AF evaluation value calculating device, the accumulators for integrating the contrast values do not cause increase in the size of the apparatus and power consumption, and deterioration in speed of the AF control due to excessive consumption of the processing power of the CPU for transmitting the AF evaluation values to the memory.
In recent years, however, there is a tendency that the larger number of AF areas are set in the image capturing area. Due to the tendency, the number of accumulators for integrating contrast values becomes too large and it causes problems such that the size of the AF evaluation value calculating device is increased, power consumption is increased, and the speed of the AF control deteriorates due to excessive consumption of the processing power of the CPU by transmission of AF evaluation values.